Production of proteins by expression in microbial systems has become a significant source of high value, medically important proteins. Purification and recovery of recombinant proteins are major considerations in the design of a fermentation process. While traditional methods of protein purification can be used to isolate a product, improved methods include the use of recombinant proteins. Recombinant proteins can be purified by affinity column chromatography, the desired component of the recombinant protein being purified by virtue of its covalent attachment to a polypeptide, which binds to an affinity matrix.
Certain systems exist for isolating proteins by the principle of affinity column chromatography.
U.S. Pat. No. 5,643,758 describes a system comprising maltose-binding protein (MBP). A cloned gene is inserted into a pMAL vector down-stream from the malE gene, which encodes MBP. The vector is transformed to a host cell, then the recombinant protein can express in the host cell. The cell lysate or media fraction is loaded to a column containing affinity matrix amylose and washed several times, then using a large amount of maltose to elute the recombinant protein.
U.S. Pat. No. 5,202,247 describes a system comprising cellulose-binding domain. A cellulose column can be used to purify the recombinant protein that contains cellulose-binding domain. The cell lysate or media fraction is loaded to the column and washed. The interaction between cellulose-binding domain and cellulose appears to be driven by hydrophobic interaction at neutral pH. The general method for elution used low polarity solvents such as ethylene glycol, prior to removal of the low polarity solvents by dialysis and filtration.
These current protein purification systems have some disadvantages. The purification processes are inconvenient and laborious. The columns used in purification are expensive. Limitations for protein purification of these systems include unable to isolate the recombinant protein in certain conditions such as EDTA-containing samples as well as the current protein tags being used are relatively large as compared to the target protein bigger than that of this invention.
There are some problems utilizing enzyme products for aquatic feed at present, such as (a) enzyme quickly flows away after feed is added into water, and (b) enzyme activity will be destructed when feed pelleting temperature is higher than 100° C.